In order to understand better the present state of the art and the problems associated with it, a cage for rolling bodies of the known type will be first described. With reference to FIGS. 1-5, a conventional cage 10 is formed by a circular base rib 11 and a plurality of circumferentially spaced fingers 12 which extend from a side of the rib 11, where the fingers 12 have a base portion 12a. The base rib 11 and the fingers 12 have partially spherical concave surfaces 14, 15, 16 defining together a plurality of partially spherical pockets or cavities 13 for retaining balls (20 or 21 in FIG. 5).
The rib 11 is a continuous structural element which extends circumferentially along the cage and forms a solid base so as to provide the cage overall with the necessary rigidity for keeping the balls of each row circumferentially equally spaced along the raceways of a bearing.
In a bearing with a double row of balls (FIG. 5), the ribs 11 of the two cages are positioned on either side with a small amount of axial play in the middle. In many applications, a bearing of this type must be axially compact. For this purpose, it has been proposed in the past manufacturing a single cage forming two rows of pockets on opposite sides thereof for two respective rows of balls. Although a reduction in the axial mass is achieved, owing to the presence of a single rib instead of two, this cage has not produced satisfactory results. As is known to persons skilled in the art, in certain working conditions, the two rows of balls of a bearing of the above type roll at different speeds owing to non-symmetrical loads acting on the bearing. In these conditions, the aforementioned single cage forces the two rows of balls to rotate at the same speed. Consequently, an excessive amount of contact (and therefore a considerable degree of friction) arises between the balls and the pockets of the cage. In the long run this results in melting of the surfaces of the pocket.
In order to solve this problem, a different cage has been proposed in EP1847725. With reference to FIG. 6, which also relates to the prior art, in a cage for ball bearings 10, a plurality of circumferentially spaced fingers 12 extend from a first side of the cage. The fingers have concave surfaces 15, 16 which form a corresponding plurality of pockets 13 for containing the balls. Each pocket 13 is defined by the concave surfaces 15, 16 of two consecutive fingers 12 and forms part of a spherical surface S. The fingers 12 have base portions 12a joined together by a continuous circular rib 11. The rib 11 extends circumferentially in a radially inner position with respect to the base portions 12a and does not project axially substantially beyond a radial plane P tangential to the spherical surfaces S on a second side of the cage opposite to the first side. This solution, although constituting an improvement with respect to the preceding solutions and reducing the friction between the cage and rolling bodies, has been unable to obtain the desired reduction of the friction.
The known solutions moreover, on some occasions, do not allow the cages to be mounted from the same side of the bearing, since the cage to be positioned in a position at a distance from the mounting side has to pass through shoulders which may be present on one of the rings of the bearing and which thus prevent the cage from passing through during the assembly operations.
There therefore exists the need to design a cage for rolling bearings which does not have the aforementioned drawbacks. In particular, there exists the need to reduce further the frictional force which arises between the cage and the rolling bodies and to allow mounting of the cages from the same side of the rolling bearing.